The tradeoff between 110 vs 220 - please explain

[Frank Ketchum]

Ok "William"... Admit to it ... you are the one we call Norm Abram. We have discovered his identity on the wreck!

Frank

[ATP]

You say that like it's a bad thing

I don't think 3 phase is anything to gloat about. I would

[William]

Dam, now I have been outed!! I feel neked where's my flannel shirt:-)

William...... AKA Norm.... :-)

[Tim Carver]

breaker using #10 wire while the 10A, 240V motor will

will be negligable difference in the performance

Nobody who cares about I^2 * R losses and voltage drop is going to take the trouble to run a motor on 240V and then turn around and cheap out by using #14 wire just because it's legal to do so. Most people would use #12 wire or better (I personally would never use #14 for anything, given a choice; the stuff is a gift to the power companies. Feel a piece of #14 romex when it's carrying 10A or so sometime - it is noticeably warm).

And if you really want to go overboard, you could even use #10 for your 10A, 240V motor, in which case your I^2R losses would be one fourth of what they would be in your example for the same #10 wire at 120V, 20A. Bottom line is, if you've got room for the double pole breaker, there is no good reason not to use 240V for your motors, it can't hurt and will always same some amount of energy over time, even if it isn't huge.

Tim Carver [e-mail]

[Robert Bonomi]

on myself consider the following:

they are wired in series.

winding and that the heating is a result of I^2 * R.

given I=E/R

at 120V that'd give a per-winding current flow of 'merely' 240A. :)

by the E=I/R rule, the 'apparent resistance' of a motor drawing 20A at 120V is 6 ohms. the 'apparent resistance' of a motor drawing 10A at 240V is 24 ohms.

The `total power consumption' is identical, agreed.

For complicated reasons, at the lower voltage you get somewhat _less_ 'shaft horsepower' output than at the higher voltage, _given_the_same_power_ _input_. The difference in mechanical power output is typically a few percentage points. (Although the motor on my delta contractor saw is rated

1.5HP @120V, and 2.0HP @ 240V. with a similar 'mis-match' in current draw at the two voltages.)

With the same power in, and lower _mechanical_ power out, it should be obvious that some power is going 'somewhere else'. Which it is. into heat.

[LRod]

Which rule is that?

If the windings in a dual voltage motor are split and are wired in series at 240V and parallel at 120V, how is the motor to know to produce less shaft horsepower when the voltage (and current) in the individual winding is the same for either source?

There shouldn't be a difference.

LRod

Master Woodbutcher and seasoned termite

Shamelessly whoring my website since 1999

[Don Mackie]

It's there to power a kiln. I do have a couple of big motors I picked up as scrap for entertainment and, since I have it, am building a linear motor which should be fun...

[GeeDubb]

I'm wearing it!

Wish i had 3-phase. :-(

Gary

[William]

Well.... You can......:-) I have a real nice ( 7.5 hp hard start, 15 hp normal ) rotary phase convertor I'm going to be selling soon as I no longer need it. Starts and runs on a 50 amp @ 240v

[Robert Bonomi]

The 'one with the typo', isn't it obvious? E=I*R I=E/R R=I/E

I was doing the math right, just got the wrong symbol in the formula. thanks for the catch.

I don't know all the theoretical underpinnings, I can state it is "observed fact" from having motors on a test stand.

The electrical characteristics _are_ different, depending on whether the windings are in series or parallel. If the windings are not _exactly_ identical, you get different voltage/current relationships depending on the type of inter-connect. In series, the current through both windings is identical, but the higher resistance winding will have a larger voltage drop across it. OTOH, in parallel, the voltage across both windings is identical, but the lower resistance winding has more current flowing through it.

"In theory, there is no difference between theory and practice. In practice, on the other hand ....."

If you want a _real_ mind-bender, try and figure out why motors run at

1725 RPM or 3450 RPM. The 'obvious' speeds are 3600 RPM, and even sub-multiples thereof (1800, 1200, 900, 720, 600, etc.), depending on The number of 'poles' in the motor design. Yet, 4+ percent 'slow' is nearly universal -- pretty much independent of any combination of manufacturer, HP, or operating voltage.

[Frank Ketchum]

In the real world, there are inefficiencies which contribute to actual performances being different than calculated performances.

Frank

[Roy Smith]

It's only a mind-bender if you've never studied how an induction motor works. There are many different ways to build a motor. The most common design that you find in shop machinery is what's called an induction motor.

The electric field created by the stator windings rotates at 3600 RPM (for a 2-pole motor). The rotor rotates a little slower than that (say,

3450 RPM). The difference, 150 RPM or 2.5 Hz, is what induces current to flow in the rotor windings (hence the name). There are no brushes. As the load increases, the rotor slows down and the slip frequency increases, causing the rotor current to increase. It's a very clever design.

One of the drawbacks of induction motors is that the rotational speed is not constant. As load increases, the rotor speed drops. For many applications, this is not a problem.